The present invention relates to a method for producing deionized water by using electro-regenerating deionization (hereinafter referred to as EDI). More particularly, it relates to a method for efficiently producing pure water or highly deionized water which is called e.g. ultra-pure water, which is used for manufacturing industries such as a pharmaceutical-manufacturing industry, a semiconductor-manufacturing industry and a food industry, or boiler water and other laboratory facilities.
Heretofore, as a method for producing deionized water, it is common to obtain deionized water by passing water to be treated through a bed packed with ion exchange resins so that impurity ions are adsorbed on the ion exchange resin and removed. In this method, the ion exchange resins having its ion-exchanging and adsorbing abilities lowered have to be regenerated, and the regeneration is carried out usually by using an acid or an alkali. However, this method has problems such as troublesome operation for regenerating the ion exchange resins, and discharge of a waste liquid due to the acid or alkali used for the regeneration.
Therefore, a method for producing deionized water which requires no such regeneration is desired. From such a viewpoint, an EDI method which requires no regeneration operation by a chemical reagent such as an acid or an alkali has been recently developed and practically used. This method employs an electrodialyzer wherein anion exchange membranes and cation exchange membranes are alternately arranged to form demineralizing compartments and concentrating compartments alternately, and a mixture of anion exchange resins and cation exchange resins is accommodated in the demineralizing compartments. Voltage is applied while supplying water to be treated to the demineralizing compartments and supplying a concentrated water to the concentrating compartments arranged alternately to the demineralizing compartments to carry out electrodialysis to produce deionized water and at the same time, to carry out regeneration of the ion exchange resins. Accordingly, with said method, no additional regeneration of the ion exchange resins has to be carried out.
A conventional EDI method employs a deionized water producing apparatus comprising an electrodialyzer wherein cation exchange membranes and anion exchange membranes alternately arranged between an anode compartment provided with an anode and a cathode compartment provided with a cathode to form demineralizing compartments partitioned by the anion exchange membranes on the anode side and partitioned by the cation exchange membranes on the cathode side, and concentrating compartments partitioned by the cation exchange membranes on the anode side and partitioned by the anion exchange membranes on the cathode side, and anion exchange resins and cation exchange resins are accommodated in the demineralizing compartments. Impurity ions in water to be treated are removed by applying a voltage to the deionized water producing apparatus, while supplying the water to be treated to the demineralizing compartments and supplying a part of the water to be treated or already treated water as the concentrated water to the concentrating compartments.
According to this method, as mentioned above, the ion exchange resins are continuously regenerated simultaneously, and it therefore has an advantage that regeneration by a chemical reagent such as an acid or an alkali is not necessary, and a treatment of a waste liquid to be generated by the regeneration is not necessary. However, there are problems with the EDI apparatus that the electric resistance gradually increases due to hardness components in the water to be treated such as calcium ions, magnesium ions and the like, thus leading to increase in the applying voltage or to decrease in electric current, and further, the resistivity of the treated water tends to decrease due to decrease in the demineralization performances.
Many methods have already been proposed to overcome such problems, and examples of which include a method of preliminarily subjecting water to be supplied for an EDI apparatus to a reverse osmosis membrane treatment in two steps to remove hardness components as much as possible and then supplying said water as the water to be treated by the EDI method (JP-A-2-40220) and a method of subjecting water to electrolysis in an acidic water-producing electrodialyzer which is prepared separately, and passing the acidic water produced in an anode compartment through the concentrating compartments of the EDI apparatus (JP-A-10-128338). By employing such methods, long-term performance of the EDI method may be stabilized, but the investment cost tends to increase, and as a result, the advantages of the EDI system as compared with other deionization methods tend to diminish.
Further, a method has been proposed in which a liquid having an electro-conductance of from 100 to 800 xcexcS/cm by adding an aqueous solution of a hydrochloride or sulfate of an alkali metal added thereto is supplied to the concentrating compartments of the EDI apparatus to stabilize the electric current in the EDI method to obtain a treated water having a high purity (JP-A-9-24374), but the long-term stability in the performance is not clarified.
The present invention relates to a method for overcoming the above problems of the conventional system for producing deionized water by EDI and the improved method for producing deionized water having long-term stability which has been proposed later. Thus, it is an object of the present invention to provide an expedient and inexpensive EDI demineralization method which can prevent and overcome decrease in performance due to impurities such as hardness components contained in the water to be treated, to be supplied in the EDI apparatus.
The present invention provides a method for producing deionized water to attain the above object, which comprises employing a deionized water producing apparatus comprising an electrodialyzer, wherein cation exchange membranes and anion exchange membranes are arranged alternately between an anode compartment provided with an anode and a cathode compartment provided with a cathode so as to form demineralizing compartments partitioned by the anion exchange membranes on the anode side and partitioned by the cation exchange membranes on the cathode side and concentrating compartments partitioned by the cation exchange membranes on the anode side and partitioned by the anion exchange membranes on the cathode side, and ion exchangers being accommodated in the demineralizing compartments, supplying water to be treated in the demineralizing compartments and supplying a concentrated water which is an electrolyte solution to the concentrating compartments under applying a voltage to transfer and remove impurity ions contained in the water to be treated, wherein the concentrated water at the outlet of the concentrating compartments has a S value of 7 or more as defined by the following formula (1) and a pH of 2.5 or more:
S value=(xcex3xe2x88x92420000xc3x97A)/(Bxc3x97(1xe2x88x92(A/0.004))3)xe2x80x83xe2x80x83Formula(1), 
wherein xcex3 is electro-conductivity (xcexcS/cm), A is hydrogen ion concentration (mol/l), and B is magnesium ion concentration (ppb).
In accordance with the present invention, the S value of the concentrated water at the outlet of the concentrating compartments (hereafter, referred to as outlet concentrated water) is maintained 7 or more, whereby can be suppressed such problems that hardness components such as calcium ions, magnesium ions and the like are bonded to OH ions and carbonate ions to form slightly soluble salts in the vicinity of anion exchange membranes on the concentrating compartment side.
In the present invention, there are generally two types of means for maintaining the above-mentioned S value of the outlet concentrated water at 7 or more.
The first means is the one that water to be treated having the S value of 7 or more, preferably 10 or more is supplied to the demineralizing compartments of the deionized water producing apparatus. The second means is the one that in the case of water to be treated having the S value of less than 7, it is supplied to the demineralizing compartments of the deionized water producing apparatus as it is without particularly being subjected to a pretreatment. However, while supplying the water as it is, concentrated Mg ions are selectively removed by using chelate resins, or a monovalent cation type electrolyte is added to the concentrated water to adjust the S value of the outlet concentrated water at 7 or more, preferably at 10 or more.
In the present invention, one or both of the above-mentioned means may be employed, whereby the performance of the deionized water producing apparatus can be stabilized for a long period of time, even if hardness components in the water to be treated are not previously removed as least as possible. Particularly, even in the case of water to be treated containing hardly ionizable impurity components such as silica and carbon dioxide gas, it is possible to operate the deionized water producing apparatus at higher current density to remove such impurity components effectively. Further, the electro-conductivity of the concentrated water in the apparatus is increased, it is possible to operate the apparatus at lower voltage to reduce electric power cost.
The above-mentioned first means will be described more concretely. The S value of raw water for deionized water, such as river water, lake or pond water, underground water and tap water, is usually 1 at highest. With the first means, the S value of such raw water is made to 7 or more, in order to being used as water to be treated for the deionized water producing apparatus. For such a purpose, may be preferably employed either of the following methods: one method wherein an electrolyte other than Mg ions is added to the raw water to increase its electro-conductivity and the other method wherein Mg ions in the raw water are selectively removed or ion-exchanged with ions other than Mg ions.
In the former method, such problems may arise that the amounts of ions to be removed by EDI apparatus will be increased to decrease its treating rate, while the electro-conductivity of the water to be treated will be heightened. On the other hand, when the latter method is carried out, Mg ions can be selectively removed by using chelate resins, or can be ion-exchanged with ions other than Mg ions, preferably monovalent cation by using so called softener as a pretreatment for EDI apparatus, either of which is a preferable embodiment. However, if such ion-exchange pass break-through point, Mg ions will be suddenly leaked with a high concentration, which will obligatorily require troublesome maintenance works.
The inventors of the present invention have studied the above-mentioned problems and have found that a reverse osmosis membrane having specific properties is preferred as a pretreatment apparatus for EDI apparatus. That is, the above-mentioned problems have been found to be resolved by employing a reverse osmosis apparatus equipped with a reverse osmosis membrane having a T value of 10 or more, as defined by the formula (2) described below as a pretreatment apparatus for a deionized water producing apparatus and by supplying the water pretreated by using such a pretreatment apparatus to EDI apparatus. In the formula (2), a NaCl removal rate and an MgCl2 removal rate, which are fundamental properties of a reverse osmosis membrane are used.
T value=(100xe2x88x92NaCl removal rate (%))/(100xe2x88x92MgCl2 removal rate(%))xe2x80x83xe2x80x83Formula(2) 
The NaCl removal rate and the MgCl2 removal rate are obtained by measuring an aqueous solution having concentration of 0.1 mass % at 0.8 MPa at 25xc2x0 C.